The proposed research is concernced with the role of cytoplasmic calcium ions in regulating excitability and other functions of the giant somata of gastropod molluscan neurons. Molluscan neurons offer unique experimental advantages while clearly exemplifying regulatory actions of calcium common to a broad range of cell types. Efforts will focus on description, mechanism, and consequences of the cytoplasmic calcium ion concentration transient induced by membrane depolarization. I have developed a microspectrophotometer capable of spatially resolving changes in optical density of the calcium indicator dye arsenazo III within the intracellular volume of living somata. The technique will be used in conjunction with microelectrode voltage clamp to describe the temporal and spatial form of depolarization-induced cytoplasmic calcium transients. This data will then be interpreted with the help of computer models to elucidate the processes which must underlie the calcium transient: voltage-dependent calcium current, cytoplasmic diffusion and binding of calcium ions, and active uptake of calcium by membrane ion pumps. One way in which calcium transients affect soma excitability is by activation of a membrane potassium conductance. Gating of the calcium-activated potassium conductance under voltage clamp will be studied quantitatively in relation to arsenazo measurements of intracellular calcium. Voltage clamp and arsenazo techniques will also be combined to investigate possible involvement of cytoplasmic calcium ions in actions of lysine vasopressin, serotonin, cAMP, cGMP, and phosphodiesterase inhibitors. Two additional techniques will be used to measure spatial distributions of calcium current, calcium-activated potassium current, and agonist receptor sites over membranes of soma and proximal axon. A vibrating probe electrode provides for localized measurement of current in voltage-clamped cells, and an external perfusion patch pipette allows test solutions to be confined to limited areas of the soma surface membrane.